1. Field of the Invention
The present invention relates to image forming apparatuses having a function of measuring color of a measurement image.
2. Description of the Related Art
Factors of image quality of an image forming apparatus include graininess, in-plane uniformity, character quality, and color reproduction (including color stability). Nowadays multicolor image forming apparatuses are prevalent, and color reproduction is regarded as one of the most important factors of the image quality.
A person has memories of expected colors (especially, human skin, blue sky, metal, etc.) derived from experiences and may feel uncomfortable if a color is beyond an acceptable range thereof. Such colors are called memory colors, and the reproducibility of the colors becomes an important factor in outputting pictures, and the like.
In addition to the photo images, also in reproducing document images, the degree of demand for the color reproducibility (including stability) to the image forming apparatuses is growing among office users who feel uncomfortable with differences of the document image colors from colors on monitors, and graphic art users who pursue color reproducibility in CG images.
In order to fulfill such users' demands for color reproducibility, an image forming apparatus is proposed which reads a measurement image (patch image) formed on a sheet with a color sensor provided on a conveyance path of the sheet (for example, see Japanese Patent Application Laid-Open No. 2004-086013).
In the image forming apparatus, measurement images are formed on a sheet, and based on a result of the patch images read by the color sensor, feedback on process conditions such as exposure amounts and developing bias is carried out. This enables reproduction at a certain density, a certain gradation, and a certain tint.
The arrangement of the color sensor for the use as the in-line sensor in the image forming apparatus, however, causes variations in the measurement values due to various factors such as attachment accuracy in attaching the sensor to the apparatus, the environment within the apparatus, and change with time. Especially, in a case where a plurality of color sensors are provided in the image forming apparatus, measurement value variations among the sensors cause difficulty in high-accuracy matching and in maintaining color stability.
Further, in a case where there are no variations in the measurement values of the color sensors arranged in the image forming apparatus, there may be differences in the measurement values among the image forming apparatuses having sensors of the same type, and this may cause tint differences among the apparatuses.
Consequently, it is necessary to check how much the measurement values of the color sensors vary to correct the measurement value variations. For the correction, it is required to measure a chart as a reference with a color sensor to calculate variations in the measurement values, and to calculate a correction value for reducing the variations in the measurement values.